1. Technical Field
The present invention relates to a constant current circuit and a flat display device, and the invention can be applied to, for example, a liquid crystal display in which a driving circuit is integrally formed on an insulating substrate. The present invention can reduce variations compared to conventional constructions by driving a transistor to function as a constant current circuit by means of a voltage of a sampling capacitor, after charging the sampling capacitor with a reference current and setting a gate-source voltage of the transistor due to the reference current in the sampling capacitor.
2. Background Art
Conventionally, in various integrated circuits, constant current circuits are constructed with current mirror circuits so as to supply currents necessary for the operations of individual sections. FIGS. 1 and 2 are connection diagrams respectively showing constant current circuits using current mirror circuits. The constant current circuit shown in FIG. 1 is adapted to fold back a reference current due to a P-channel MOS (hereinafter referred to as PMOS) Q1 by means of N-channel MOS (hereinafter referred to as NMOS) transistors Q2 and Q3 based on a current mirror circuit construction, thereby causing a constant current corresponding to the reference current to flow out from a desired circuit block, whereas the constant current circuit shown in FIG. 2 is adapted to fold back a reference current due to an NMOS transistor Q4 by means of PMOS transistors Q5 and Q6 based on a current mirror circuit construction, thereby causing a constant current corresponding to the reference current to flow out from a desired circuit block.
In recent years, a liquid crystal display device of the type in which a driving circuit for a liquid crystal display panel is integrally integrated and constructed on a glass substrate which is an insulating substrate constituting part of the liquid crystal display panel has been provided as a liquid crystal display device which is a flat display device applied to mobile terminals such as mobile phones and PDAs. In such a flat display device as well, driving circuits use the constant current circuits mentioned above in the connection diagrams of FIGS. 1 and 2.
Specifically, this kind of liquid crystal display device has a display section formed by pixels which are arranged in a matrix form and each of which is made of a liquid crystal cell, a polysilicon TFT (Thin Film Transistor) which is a switching device for the liquid crystal cell, and a storage capacitor, and it is constructed to display various images by driving the display section by means of various driving circuits arranged at the periphery of the display section. As disclosed in Japanese Patent Application Publication No. Hei7-295521 and the like, the liquid crystal display device is constructed to drive the respective pixels by means of analog signals based on gradation data indicative of gradations of the respective pixels after setting signal lines of the respective pixels to a predetermined potential and charging and discharging their storage capacitors by precharge processing, for example, on a line-by-line basis, and such a constant current circuit is provided in a circuit block or the like associated with driving using the analog signals.
However, TFTs which are active elements applied to this kind of liquid crystal display device have the disadvantage of large variations, and if the constant current circuits shown in FIGS. 1 and 2 are constructed by using transistors which are such active elements, there occurs the problem that the reference currents set by the transistors Q1 and Q4 vary, and in addition, a current which flows into the transistor Q3 and a current which flows out from the transistor Q6 vary with respect to the respective reference currents.
In order to reduce the influences of such variations on individual circuit blocks, heretofore, the design has been to cause comparatively large currents to flow in constant current circuits using this kind of active element. However, this design offers the disadvantage of increasing power consumption by that amount.